46 Proceedings of the Royal Irish Academy. 
by phj^siologists, and it numbers among its supporters the most distin- 
guished philosophers. 
Dr. Brown-Sequard conceives 'Hhat he has ascertained that, beside 
the four distinct kinds of nerve fibres of the higher senses, there are at 
least eleven kinds of nerve fibres in the spinal cord, and in the cranial, 
spinal, and sympathetic ner\^es." 
He enumerates these eleven kinds as follows : — 
1st. Conductors of impressions of touch. 
2nd. ,, ,, of tickling. 
3d. ,, ,, of pain. 
4th. ,, ,, of temperature. 
5th. ,, ,, of muscular contraction. 
6th. Incito-motor conductors. 
7th. Incito-nutritive and secretory conductors. 
8th. Yoluntary motor conductors. 
9th. Involuntary motor conductors. 
10th. Vaso-motor conductors, 
nth. JS'utritive and secretory conductors. 
I hardly need say," he adds, ''that the number of functionally dis- 
tinct nerve fibres is probably much greater than is shown in this 
table." 
As regards the physiology of sensations of colour, a theory so closely 
analogous as indeed to be identical with reference to the sense of vision 
was put forward by Thomas Young, at the commencement of this cen- 
tury. He supposed three sorts of conductors to exist in the optic nerve, 
each specially charged with the function of conducting a diff'erent colour, 
red, green, and violet. The mixture of these three colours in different 
proportions gave rise to all the other colours of the spectrum. 
This hypothesis of Young has, with some modifications as to the 
colours, found a zealous advocate in the distinguished Professor Helm- 
holtz. 
It is not necessary for my purpose to enumerate the various theories 
which have been advanced in explanation of the various phenomena to 
which I have just alluded. Suf&ce it to say that I have long felt that 
the ingenious idea of distinct conductors did not exactly meet the case. 
So long ago as in 1861, in a critique on Dr. Brown- Sequard's work in 
which his theory was first put forward, I expressed the opinion that we 
could hardly accept the idea " that the nerve fibres employed in the trans- 
mission of sensitive impressions of touchy ticlding, pain, &c., are as dis- 
tinct one from the other as they all are, from the nerve fibres employed 
in the transmissions of the orders of the will to the muscles." 
The theory which I venture to propose, and which I put forward 
with diffidence when I consider that another has been advocated by 
such able physiologists as Helmholtz and Brown-Sequard, is simply an 
application of the theory of wave propagation to the passage of various 
sensations along nerve conductors. 
I conceive that the various peripheral expansions of sensitive nerves 
